Scientific Investigations Report 2008–5071
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5071
Conceptual Model of Hydrologic and Thermal Conditions
The hydrogeologic framework of the Eastbank Aquifer system consists of the Upper and Lower Aquifers, which are highly permeable sand-and-gravel aquifers separated by the Clay Confining Unit (fig. 5). In the northwestern part of the study area (fig. 6), the Clay Confining Unit is absent and the Upper and Lower Aquifers merge to form the Combined Aquifer. The lower boundary of the Eastbank Aquifer system is crystalline bedrock that consists of biotite gneiss with low permeability. The bedrock has an undulating surface that forms a basin in the central part of the study area that is deepest near the RW well field (fig. 7). The Lower and Combined Aquifers in the eastern part of the study area are truncated by the bedrock surface (fig. 8) and the Clay Confining Unit and Upper Aquifer continue farther to the east, where they also are truncated by bedrock (fig. 5). The southern boundary of the aquifer system is a subsurface cutoff wall that is a partial barrier to ground-water flow from the Upper and Lower Aquifers. The northern and western boundaries of the aquifer system are the Columbia River. Along the western boundary, the Lower and Upper Aquifers are truncated by the river, and along the northern boundary, the Combined Aquifer extends beneath the river for an unknown distance to the north.
The Upper, Lower, and Combined Aquifers primarily are recharged by water from the Columbia River. Along most of the western boundary of the aquifers, ground-water recharge occurs across a layer of fine-grained, low-permeability sediments. Discharge from the aquifer system is ground-water pumpage from the Lower Aquifer and ground-water seepage from the Upper and Lower Aquifers around and through the subsurface cutoff wall.
During post-dam, predevelopment conditions, ground water generally flowed from the northeast to the southwest, approximately parallel to the river (fig. 11). With the onset of significant pumping from the Lower Aquifer by the RW well field in 1983 and the CT well field in 1989, two overlapping cones of depression have formed in the Lower and Combined Aquifers (fig. 15) and an approximately east-west trending ground-water divide has formed between them. The location of the ground-water divide probably varies slightly over time depending on which wells are pumping and at what rate. In 2006, mean annual pumpage from the RW and CT well fields was about 16 and 43 ft3/s, respectively. Pumpage from the SW well field from the Lower Aquifer and from the LR well field from the Combined Aquifer is small compared to pumpage from the RW and CT well fields and has a negligible effect on the ground-water flow system. Because of the hydraulic properties of the Clay Confining Unit, water levels in the Upper Aquifer are assumed to be relatively unaffected by pumping in the Lower Aquifer. Data do not exist, however, to confirm this assumption.
The cone of depression in the Lower and Combined Aquifers surrounding the RW well field draws water primarily from the west and secondarily from the north (fig. 26). An additional, smaller amount of water is drawn in from the south and east and, presumably, from beneath the wells. The cone of depression surrounding the CT well field draws water primarily from the west and southwest (fig. 26). An additional, smaller amount of water is drawn in from the north and east. Any water in the Lower Aquifer south of well CT3 not captured by pumping becomes seepage through the subsurface cutoff wall. Because of its proximity to the ground-water divide between the two cones of depression and because the location of the ground-water divide may shift as pumping patterns change, some of the water pumped by well CT4 may originate from a bedrock depression to the north and west.
Most of the Lower and Combined Aquifers have been in thermal equilibrium since 1999 and this equilibrium was reached during 1991–98. The only exceptions are the Lower Aquifer near wells TH1 and TH4, which reached thermal equilibrium prior to 1991, and the Combined Aquifer near well LR2-W, which had not reached equilibrium by 2006. At thermal equilibrium, the time lags between changes in river temperatures and subsequent changes in ground-water temperatures are constant at a given location and the ratios of annual temperature ranges in ground water to annual temperature ranges in the river also are constant at a given location. Because time lags and annual temperature-range ratios vary in three dimensions, the Eastbank Aquifer system is a mosaic of different temperatures at any time of the year. Generally, however, time lags increase and annual temperature-range ratios decrease with distance from the river.
Mean annual minimum and maximum temperatures of source water in the Columbia River that recharges the aquifer system were 2.5 and 19.2°C, respectively, from 1991 through 2006. Typically, the annual minimum temperatures occur in February and the annual maximum temperatures occur in August or September. From 1999 through 2006, there were statistically significant increasing trends in mean annual and annual maximum river temperatures but there were no trends in the annual minimum temperatures. The increases in river temperatures resulted in a corresponding increase in Lower and Combined Aquifer temperatures, except near well TH8. Temperatures in this well may not have increased because they represent a part of the Lower Aquifer minimally affected by pumping at a greater distance from the river or because nearby well CT4 may pump colder water that may have settled locally in the bedrock depression north and west of well TH8. There were no trends in the annual minimum, mean, and maximum river temperatures from 1991 through 1998 and from 1991 through 2007. The mean annual increase in the annual mean and maximum river temperature from 1999 through 2006 was 0.07 and 0.17°C, respectively.
The dependence of the detection of river-temperature trends on the period of the record selected for analysis indicates that although mean annual river temperatures may increase during multi-year periods and these increases result in corresponding increases in the Lower and Combined Aquifers, the increases in river temperatures and thus ground-water temperatures over relatively short periods of time are within the natural variability of the river temperatures and decreases in mean annual river temperatures are likely during other multi-year periods.
Interannual trends in ground-water temperatures are controlled by interannual trends in river temperatures, interannual trends in seasonal pumpage patterns, and the extent of thermal equilibrium in the aquifer system. From 1999 through 2006, seasonal pumpage patterns were relatively stable and most of the aquifer system was in thermal equilibrium; thus reported trends of increasing temperatures of water pumped by the CT well field are most likely explained by increasing trends in river temperatures.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5071
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